Nonlinear optical amplifier



INVENTOR. BY m af QM WMZ/@M ATTORNEYS April 14, 1953 c. J. T. YOUNG NONLINEAR OPTICAL AMPLIFIER Filed July l2, 1947 BRIGHTNESS OF IMAGE ily' Patented Apr. 14, 1953 y.UNI'I'ED vSTATES PATENT OFFICE `This 'invention relates to optical devices having particular application as viewing devices for obviating Iheadlight glare in driving motor `ve- 'hlcles at night and undersimilar conditions of difficult vision.

-It'has long been recognized'thatthe glare resulting from' the headlamps of approaching vehicles constitutes a majcrhazard when driving at night and severely complicates the provision of adequate lilluminations for proper visibility. A vdriver obtains -visibility in the absence of an apl'proaching vehicle primarily by reflection of the tively low level-of brightness. This is usually suivficient for fairly gcodvisionin the absence of an kapproaching vehicle or other sources `oi direct light Yof -a high level of brightness, but when such light lof Vhigh brightness is-present, it is no longer -feasible Yfor the eyes to-make adequate adjustment to maintain proper visibility for the portions of the =field of both high andloW brightness necessary to be viewed.

Ingeneral, visibility will be improved by any "reduc-tion in the Yapparent brightness of approaching headlamps or other source of direct or glare light without a comparable reduction in the'apparent `brightness of the remainder of the field. At thesame time, it will be noted that the desired results cannot be obtained by general 0reductionin overall bright-ness 'by means of such llil-tering media assunglass-es. In fact, the use of such-media is likely to'be unsafe, since the headlamps are-ontheaverage so much brighter than -theerest of thef'eld thatto reduce their apparent r@brightnessmaterially it is necessaryto use a filter of such `low transmission properties that the remainder of thefield will be too dark for adequate perception.

It hasbeen proposed to accomplish the desired -result by providing Yautomobiles with light-polarlizing headlamps and yproviding the drivers of these cars with light polarizing -visors or eyeglasses. Such a Vsystem -results in reducing the apparent brightness of fthe lfieldby a -relatively Asmall factorlin--comparison withthe much larger reduction in .the apparent brightness of the headlamps of approaching cars. `However, this -systemmequires complementary equipment on all yehicleson-theroadfforyfull eiectiveness, andthe light .polarizing-yisorprovides no beneiit for the v.wearer aganstafcarinot .equipped -.with light.- ;polarizing headlamps One of the principal objects ofthe presentinvention is to provide a viewing device for use when driving at night or under similar. conditions of low general illumination'but'high opticalcontrast which will be effective to amplify the Vapparent brightness of the users field of view=suf ciently for clear general visibilitywhile vat lthe same time preventing the apparent brightness of the sources of direct or glare'light'from reaching a level sufficiently high to interfere with inadequate visiblity of the field as a whole.

Another object is toprovide suchva viewingfde.- vice which will operate efectively for reducing the glare of bright light sources such as approaching headlamps at night while affording adequate visibility of the remainder of the-field of view without the necessity of special ycomplementary or supplementary equipment in or-on the other vehicles and notwithstanding the apparent brightness of such direct light sources.

An additional object is lto provide a viewing device capable of receiving light rays fromaeld of view and forming an optical image of the field having amplified general apparent brightness in comparison with the field but which will 'have the characteristic of becoming selectively saturated in accordance With the distribution of brightness in the'field and of correspondinglyre'- ducing the optical contrast of the image with relation tothe field.

Still another object is to provide an `optical system for producing an image of a field of-i/'ieuf which is eiiective to vamplify the apparent brightness of the image with respect to that of thefield and which is also effective to produce predetermined alteraticn in the op-tical contrastcf .the image with respect to that of the field and thus to make predetermined areas of the image rela,- tively more visible than they are in the iield under existing conditions of illumination.

It is also an object of the invention tocprovide such a viewing device which will not interfere with the normal binocular Vision and depth perception Yof the user and which will accordingly enabletheuser to vsee clearly and without eye- .strain while protecting him against .glare from direct or other bright light sources in theilelid or image.

A further object is to provide such a device which is of relatively light weight and simple construction andwhichcan be'readily incorporated in Va. self-contained unit adapted for wear byan automobile driver or other individua-l 'without discomfort.

A *Still further object is-tonrovide such aview.-

^ ance vwith the invention; and

image. vides an image of the field wherein the bright ing device which does not require sources of artificial illumination and which affords adequate visibility for the user even under conditions such as the substantially total absence of visible light, heavy fog, or the like.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.

In the drawings- Fig. 1 is a view in side elevation of -an automobile showing the driver equipped with a binocular viewing device constructed in accordance with the present invention;

Fig. 2 is a diagrammatic viewrof one form of optical system in accordance with the invention which may be incorporated in each barrel of the -the operation of the optical systemV of Fig. 2;

Fig. 4 is a diagrammatic view illustrating a :modified construction of optical amplifier for use in the device of theinvention;

i 'Fig. 5 is a diagrammatic View illustrating -another modified form of optical amplifier for use in the device of the invention;

Fig. 6 is a diagrammatic view illustrating another modified arrangement for limting amplification in the device of the invention;

Fig. 7 is a diagrammatic view of another form of optical amplifier and optical system in accord- Fig. 8 is a diagrammatic view illustrating another modified form of optical system for use in the device of the invention.

The invention has particular application to the .provision of adequate illumination and visibility Vunder conditions of low general illumination but high optical contrast, such as are typically exemplified vby the conditions commonly encountered when driving an automobile at night.

lI'he strong light sources in a driverfs field vof view, and particularly the headlamps 'of approaching cars, are so much brighter than the remainder of the' field that theyseriously ob- 'scure general vision both while they are approaching and even for an interval after they .have passed, as is well known. The present invention overcomes these conditions of glare by providing a viewing device comparable to a goggle vwhich may be worn by a driver at' night and l which is of such optical characteristics and properties as to reproduce for the user a three-.dimens sional optical image corresponding accurately to the eld as viewed directly but of changed optical contrast, with the sources of direct or glare lightsubstantially reduced in apparent brightness and with the apparent brightness of the re- Aas to form an image corresponding to the field,

but the device is so constructed as to become selectively saturated in accordance'with the Vdistribution of brightness in the eld and thus to limit the maximum apparent brightness of the The device of the invention thus proareas are held to a maximum brightness level which will not interfere with adequate perception of the less' bright areas, and wherein in turn the apparent brightness' of the less bright areas is amplified with respect to the brightness of the corresponding areas in the field and these areas are accordingly made more readily visibile. Thus the invention provides for forming an optical image of reduced optical contrast but amplified general brightness such that each area is clearly visible while at the same time no area is so bright as to interfere with adequate perception of any other area or areas. u

'I'he invention is applicable generally to optical systems which include means vfor receiving light from a eld of View, a screen or other means for reproducing a visual image of the field in response to'light incident on the receiving means, andanoptical amplifier effective to amplify the brightness of the image withrespect to ythel brightness of the field. If the amplifier is suliiciently powerful, such a system will provide adequate visibility under substantially any conditions of low general illumination and low optical contrast. However, if there are sources-of direct or other bright light in the field, their apparent brightness will be amplified in the image to the same degree as that of the less bright areas,V with the result that the image produced by the'device will usually not afford a useful improvement in visibility over direct viewing of the field. Some areas of the field having brightness below threshold for'the eye may be made visible in the image and thus relatively morevisible than before, but this is not likely to be a sufficient imprcvement, particularly in View ofthe fact that the tendencies towards glareA from the light sources in the field will be correspondingly amplified in the image.

The present invention accordingly provides for selectively controlling amplification in an optical system of the above type in order to provide for predetermined alteration of the optical contrast in the image with respect to that in the field, with the optical contrast, i. e., the ratio of brightness of the relatively more and less bright areas, being eitherV decreased or increased in the image depending upon the desired conditions and results. For example, the invention provides for so controlling'amplification as to prevent the brightness of any area in the image vfrom exceeding a predetermined maximum level while still giving adequate amplification of the areas in the image corresponding to the less bright areas of the field. This result may be produced in accordance with the invention by producing selective saturation of the device in accordance with the distribution of brightness in the field and thus limiting the maximum apparent brightness of any area in the image. In this way, the optical contrast of the image is changed with respect to that of the field, and in particular the contrast of the image is limited, preferably relatively sharply, when a predetermined level of brightness or contrast is reached in the final image or in the initial scene or field of view. When such a level is reached at a given area of the image, further increase Vin brightness in the corresponding area of the lscissae represent the brightness, in any suitable units such as foot lambertsof the light from a fgiven :area ofipthefield of viewzincldent4 on the receiving means-of .the device, and the ordinates --represent the :apparentbrightness .in the same units ofxthe correspondingarea of the image pro- .fduced-.by .the device. It will be noted that the l'amplification curve is initially Asubstantially Jinean'iasfshown by the solid.` line .arrow,..and in vIthefabsence of previsioniforxsaturaticn, thecurve could be expected to .continue proportionately vlinear as indicatedby the dotted line arrow. However, the invention Vprovides for'preventing 'thisi-linear'function' and for progressively limitingy amplification up'to a predetermined 'saturartion value at which the amplification curve levels .o1rapidly'and thereafter remains substantially invariantwithzthe input as shown'in Fig. 3. This 'saturation .may occur at a fixed .brightness vlevel :in either the eld or the-image, with the amplication being adjusted to bring the maximum .brightness ofthe image other than direct light sources'therein to alevelLrelatively closely below .thevmaximum or it may occur at a sliding level related to the average brightness of the scene or 'leld of vi'ew'as a whole, and it will accordingly be understood that the curve in Fig. 3 is `merely l illustrative of conditions attainable by meansof the present invention.

'.In .the use of the invention for night driving or under similar visibility and illumination ccnditions, it provides for selectively controlled amplication of the light from a iield of view to give an optical image of. amplied general brightness Vbut reduced'contrast' and limited maximum brightness, with the reduction in contrast being moreor less inV inverse ratio to the contrast of the field as viewed directly. Thus for example, if the contrast in the field is low, as would be 'likely at night in the absence of artificial light :sources,.the image produced bv means of the invention would possess substantially the-same conl trastas the field but a generally higher level of brightness. On the other hand` the field of View ofian automobile driver when another car is approaching is one of extremely high contrast. with the headlamps of the other car being relatively very much brighter than the remainder of the eld. The invention provides a device capable of use under such conditions to produce an image .of correspondingh7 reduced contrast. Thus the device lwill produce an image wherein the areas surrounding the lheadlamps are of amplified brightnessv but with the headlampsthemselves of reduced 'apparent brightness not onlv in comparison with'the surrounding areas of the image but also in comparison with their brightness viewed directly in the field, or the `headlamps themselves may preserve their actualbrightness in the image or even beincreased in apparent brightness provided the rest of the field is amplilied toa suiiiciently greater extent to decrease the image contrast sufficiently for'adequate pereeption and visibility.

Referring tc the drawing. which illustrates preferred embodiments of the present invention, Fig. 'l shows an automobile I0 provided with headlights Il and with the driver wearing a binocular Viewing device I2. In accordance with the invention, the viewing device I2 includes a pair of optical systems, one for each eye of the wearer, for receiving light from the wearers iield of view and reproducing an image of the field of changed opticalcontrast such that those areas of the field which constitute sources of direct or glare light'are reduced in apparentbrightness in the -imageand .are thus prevented from inter- :ieringI ywith iadequate visibilityy Iof the'rsurround- .ing orother areas' or :parts "of the image.

`In addition, .these a two systems fare-so varranged in the viewing .devicethat their respective means forrreceiving light from the-.field are spaced at :substantially the .average `interocular distance :to provideathree-dimensionalimagehaving the rproper 'accurate-relation to .the ',eldfa's -viewed directly.

Fig. 2 illustrates'an optical systeml in accordance vvith'the vinventionv whichwill provide these :desired results when incorporated in a viewing devicefsuchas the binocularr device l2 in zFig.1. This system includesa layer`20 of photoelectric material zona supporting layer-'f2 Lia layer 22 of ifsecondary .electron emissive :material on .arsupportin'gtlayer 2-3`in the ipath ofprimaryi'eiectrons from layer: 20, and' anielectron lensM'fadapted to focus'the electrons from slayers' 202and^22 .on

-a suitableviewing screen125 suchiasa phosphor screen. .An electricield iszgprovided asindicated to draw the electrons'through"the.;system,"'the spaces between the. electronfemissive Alayers .and

.the screenbeing evacuated suiiicientlyito allow the electrons to complete their Vtrajectories-sub- .Stantially without interferencefromf gas .molecules. *A wide-angle lens V26 may bepositioned as shown inFig. Zbe'tween photoelectric receiving layer 2G and the fieldiof view, and itmay be vprovided with a suitable stop `2"!'to control stray light andA reduce aberration. The image on screen '25 may be viewed througha'suita'bleeye piece 28; asishownpanda suitableenvelope'for housing lis Vindicated at'29. lIt shouldgalso be notedv that for some purposesrof the invention, the layer'20 should be sensitiveto invisible light rays such :as'inrared or 'ultraviolet as well as to light rays Ainthe visible portion of the fspectrum. Such invisible light .raysmay be :presentin substantial quantity when-visiblellight is at a minimum; since infrared rays have substantial Apenetrating '2li may in turn dislodge a plurality of secondary electrons from the ylayer 22. In additiomif the supporting layer 23 be sufficiently thin, for ex- .amplea thin pellicle offtransparent plastic or a .thin cleavage plate of mica,.,mostof'the primary electrons `will 'be able .to penetrate it and lthus 'be ableto-reach'the emissive layer 22. vThe parts arepreferably arranged 4as Yshown in Fig.`v2, With the layer 22 on the surface of supportingllayer 23 facing the screen 25, Vbecause the secondary electrons will be emitted with relatively' low velocityand will be less able to penetrate vthe support 23 than the primary electrons. The electric potential of the source of Secondary electrons may be between those of the primary source and the screen, as by means of voltage-dividingv resistors 30 and 3| as shown. YIt should be also noted that optimum image clarity is obtained if the secondary source is near an electron image surface, i. e., near either the primary source or the screen, particularly in systems of the type shown in Fig. 2 and including an electron lens.

The desired saturable amplification maybe obtained with the system illustrated in Fig. 2 .by forming one or both of thelayers .20..and 22 from which electrons are to be emitted of such thickness as to limit the number of electrons available. Particularly effective results .will be obtained by forming suchlayer or layers-cfa ma,-

' electrons.

terial having appreciable electric resistance, for example a thin film of a metal such as caesium or other material suitable as a source of photo- With this arrangement, if a large number of electrons are dislodged from a relatively small area of the layer, as will be the case for an area corresponding to a very bright part of the eld of view, electrons will not immediately replace those which have been emitted, and net positive charges will appear, varying in strength in accordance with the relative brightness of the corresponding area in the eld. These charges will act on the subsequently emitted electrons so that the slower such electrons will be drawn back into the emissive surface. Furthermore, thesev charges will accumulate most rapidly in the areas corresponding to the brightest areas in the field, thus providing for a correspondingly rapid increase in the limitation of eiective emission from such areas until a saturation level is reached at the maximum brightness for which the device is designed.

The graph in Fig. 3 illustrates an amplification curve and saturation effect which may be obtained by means of this system and arrangement. When the light from the eld incident on layer 20 is of low intensity, as will occur in the areas of the receiving layer corresponding to dimly lighted areas of the eld, there will be a correspondingly low emission of electrons from layer-20, and hence also from layer 22. Since this in turn will produce only weak positive charges at these areas of the emissive layers,

there will be a correspondingly slight limitation on electron emission, and the amplification will approach the maximum afforded by the system, as indicated by the straight portion of the curve in Fig. 3 nearest the zero position. On the other hand, in theareas of rlayer 20 Where the incident light is brighter, the positive charges will accumulate more rapidly and hence will increase the limitation on electron emission, and the amplification curve will fall oli at a correspondingly rapid rate until it reaches the saturation point represented by the horizontal portion at the right of the solid line curve in Fig. 3.

The particular value at which substantially absolute saturation is reached depends on the geometry of the system as a whole, the accelerating voltage applied to the electrons, and to a lesser extent on the nature of the electron emissive layers and the wavelengths of light and velocity of electrons involved, and it can therefore be readily controlled to suit desired viewing conditions.

In the above example, there is some tendency for electrons to flow within the emissive layers into areas from which substantial emission takes place, to replace the emitted electrons, and if such an area is particularly bright, this flow of electrons from the surrounding parts cf the layer maybe so great as to create a shortage of electrons for emission from the surrounding areas, which could be objectionable if there were 'any considerable illumination in the corresponding parts of the iield. This result and eiect may be controlled by forming one or more of the electron emissive layers of a multiplicity of discontinuous areas, and Fig. 4 illustrates a composite structure including such a discontinuous layer which may be substituted for either of the pairs of emissive and supporting layers 2li-2| or 22-23 in Fig. 2.

In the 3-ply structure shown in Fig. 4, the middle layer 40 is composed of material offering substantial electric resistance, for example a poorly conducting metal or metallic sulfide or a thin layer of a suitable plastic, which if necessary may contain a small amount of electrolyte to prevent its resistance from being excessive. On the side of the resistance layer 49 adjacent the iield to be viewed is a layer 4I of relatively good electric conductivity, for example a thin ,lm of a highly conducting metal or metallic substance or a lm of glycerine or a hydrophilic plastic containing a small amount of water and an electrolyte and provided with a cover glass to prevent evaporation of liquid therefrom. Onthe side of the resistance layer 40 adjacent the screen is a layer 42 of electron emissive material'which may be thin and continuous, but sharpest lim'- itation of saturation as between adjacent areas having a, marked difference in illumination is obtained if the layer is composed of a multiplicity of discontinuous areas, as shown in Fig. 4. This composite layer may be substituted either for the layer 2li-2| in the system of Fig; 2, in which case layers 48 'and 4| should be transparent to light ofthe wavelengths to which layer 42 is photosensitive, or for the layer 22-23 -in Fig. 2, in which case layers 40 and 4I should be transparent to electrons moving at Vfairly high speeds but need not be transparent to light. vAlso this composite layer may be positioned with the layer 42 facing the source of light or electronsby which it is actuated provided layers 40 and 4| are suiiiciently thin to permit the passage of some of the secondary electrons emitted from layer 42.

In operation the composite layer illustrated in Fig. 4 provides a selectively emissive source of electrons, which may be primary or secondary depending upon its relative position in the 'system. In either case, eachrdi'sco'ntinuous area of the emissive layer 42 is 'supplied with'electrons only through the part of the resistance layer 40 substantially directly in line therewith'.Y The resulting emission from areas having strong positive charges tends to be drawn back,l thus limiting effective emission from 'such areas, which correspond to the bright areas'of the eld. The discontinuities in la'yer 42 are preferably sutil- 'ciently iine as to lavoid gaps in the nal image,

satisfactory dimensions being comparable to those of an optical grating, but it is to be noted that large 'areas with iine separation' between them will 'not produce the samelack of dennition as in a television image of large elements,

since the loss will be only'in the sharpness withwhich saturation is conned and maintained within the' brightly illuminated regions. Such a discontinuous layer suitable for use for the purposes of the invention may be produced by ruling in two dimensions on a continuous layer with a ruling engine such as is used to'produce optical gratings, or it may be produced by evaporating or sputtering through a screen. y

In operation with the device as shown in Fig.

4, since the supply of light or electrons is from the conducting layer 4I to the electron emissive layer 42 through the portion of the resistance layer 4U directly therebetween, a portion of the electron emissive layer correspondingto a Abright area in the eld will acquire a relatively positive potential as fa. result of the potential drop in the resistance layer, but this will not have a substantial effect on the potential of the adjacent parts of the electron emissive layer, such eiiect being least if the thickness of the resistancelayer 40 is small in comparison with the width and separation between individual areas "assumes of' the electron emissive layer 42.. In: thislway, emission from eacharea` of the electronemissivelayer can be limited tof a predetermined maximum, with resulting control over the max'- imum brightness in each corresponding area of the image on the screen, but'` at the same time the local charges can leak away' through the layer 4G to provide fory changingA conditions of brightness m the field.

Another way in which to controlv and. reduce the optical contrast o' the image with respect to. that ofthe field is to provide nearA an electron` emissive layer a layer of material of suchv electrical characteristics as to stop some oi" the emitted electrons and thereby to acquire' a lo cally distributed charge capable of repelling or opposing additional electrons from; the portions of 1 the emissive layer corresponding-e tothe brightest parts' of the field. For example, the layer 22` in Fig. 2 may representy such arselecti've 'oppositionl'ayer positioned,y asv shown, in front of the layer r of elect-ronl crn-issive materia-i'.V The opposition layer in this arrangement may be provided with a discontinuous structure` such as: that shown-alt 50 inl Fig. 5, composed of areas which are of low transparency to electrons of: the veloci-ties in question interspersed with areas 4which. are ope-n or otherwise ofsubstantially higher transparency to suchelectrons, thelayerv 5U- being mounted ona suitable supporting layer 515. For example, such a discontinuous layer may be made by a photographic process in which, in a thinemulsion, a dense silver imageofthe desired' lowtransparencyareasv is produced, with the unreduced silver then being removed ffrom the higher transparency1 areasy by' 'xing. It will be understood that the areas' Friiv of low transparency should be r electrically con'- nected to. permit necessary leakagev of' accumulated local charges, which may be done by leaving connecting areas of higher transparency which include sufficient silver or other conducti'n'g: material. For example7 if this layerl is'madev photographically as stated', sufficient dissolved electrolyte may be permitted to remain in the layer` for adequate conductivity. The selective oppositiony layer of Fig. 5: may also be` constructed in moreY elaborate form comparable to that shown in`- Fig. 4, in which case the continuous or discontinuous silver layer will corre'- spend tothe layer 42, the layers 40 andh 4l being or thev same characteristics as described herein.- above in connection withy Fig. 4.

' In. practicing the invention with such opposizti'on layers, it will ber seen that light. from a strongly illuminated area inthek field will cause the-emission of many electrons from the receivingl photoelectric layer, Some of thesey electrons will pass through the opposition layer, but others will be stopped thereby and will produce a corresponding negatively charged area. This. chargeA will in: turn tend to repel or oppose subsequent, electrons emitted from the receiving layer,k thus reducing further transmission toi the screenand` so reducing the relative brightness of the corresponding area on the screen, and since this effect will be strongest where the number of. in lsidentv electrons is high it will. result in reducing effective emission from the. emissivelayer in the areas where it ismost concentrated, A sim-- ilar arrangement may be provided in conjunction with each of one or moresecondary emissive layers and will operate ina similar manner to produce. saturated amplification.

PA, converse result may be obtained with asturilar arrangement-of the above intercepting layer which is alsol a. source of secondary electrons; In such case, if` the primaryl electrons are fast enough, each willr cause the emission of' several secondary electrons, adequatel acceleration being dependent upon the electrical potential chosen. If thus. the. secondary electrons emitted exceed the; primary electrons stopped by the layer, the result. will be. a net positive chargeY opposite the correspondingarea of the. primary source layer, which will. reduce space charge and increase the effective emission, and. will have the ultimate resultof making. the corresponding area in the image relatively still brighter than. in the eld.

Saturasble. ampliiication` may be. obtained in accordance. with the:` invention by providing meansfor producing selective local charges on or adjacent the screen and thereby reducing the number of electrons strikingV the negatively charged areas or reducing their velocity. In this case, however, some of the electrons may be deviated instead of being entirely repelled and. may strike another part of the screen and thus produce blurring of` the image. This effect may be reduced .by providingy closelyy adjacent the screen. a source of secondary electrons and a short dis. tance in front of it a layer. or grid to receive. them, with suchv layer or grid being adapted to the accumulation of local charges as. described above in connection with 2.4 Such: anarrangement is shown in Fig. 6,` wherein the screen: 60 is. mounted on ay supportzlil andrhasadjacent its surface toward thev field of View a layer 62 of material providing asourcexof. secondary electrons, and. wherein a grid 63v is shown as positionedfcloselyadjacent ,but spaced from the layer 62. This grid. layer |53-V may have a potential in the quiescent state somewhat.A aboveA that of the screen andV layer 62; and it'may be adapted, as described above, to accumulate local charges corresponding to the image and thereby to repel electrons corresponding to the brightest portions' of the eld. This combination` of layers may be substituted for thescreen 25 in Fig. 2, and ifV the'k layer 63` is. somewhat more positivev than the screen and is spaced a short. distance in frontA of. the screen,y both the number of deviated electrons striking the screenand: also their velocity' will be substantiallyreduced.

Control of the optical contrastv of the imagey may be. obtained also by suitable, control of the thicknessuor responsiveness of the screen itself. For example, inI the. case of a phosphor` screen, the. coating of phosphor may beso thin that its f maximum brightness Will be produced with lessthan the maximum. available number and' velocity of incident electrons; Also, the thicknessorconcentration of.y the screen, as Well as the thickness or other. properties of one or more of the electron emissive: or resistance layers, may vary over the viewing area to provide for control of the. optical contrast in different areas of the image For: example, there may be a; graded increasein overallV sensitivity in the center of the viewingA area sov that each part. of the field will more may be curved if desirableto simplify problems of design of the associated optical equipment. In the case ofthe application of the invention to a viewing device specifically intended for night driving, it is desirable that the construction provide a wide field of view, a magnication factor of unity, natural interocular distance for normal depth perception, and effective viewing points as near to the drivers eyes as possible in order to make the apparent spatial relationships in the image correspond with the real such relationships by which the driver is accustomed to operate in daylight. All' these objectivesmay be readily obtained 'by suitable control of the lensesindicated schematically in Fig. 2 in accordance with standard opticalprinciples and by embodying the vsystem in a binocular mounting as shown in Fig. Land Fig. 7 shows another optical system in accordance with the invention which is particularly adapted for binocularuse. Y

In Fig. 7 the optical amplifier 65 is an enclosed receptacle of glass or other suitable material having a layer 66 of photoelectric material on its inner surface adjacent the eld of view and a fluorescent screen 61 on its opposite inner surface, and it will be seen that the device is of such dimensions as to extend across lboth eyes of the user for binocular vision of screen 61. A housing 10 for the device is accordingly shown as provided with suitable temples 1| for mounting in operative position as indicated in Fig. 1. The space 13 within the device is evacuated, and one or more layers 14 of-secondary electron emissive material may also be provided between the layer 66 and screen 61 to increase amplification. The objective lenses 15, stops 16 and lenses 11 correspond to the Vsimilar members in Fig. 2, and eyepieces 11a are provided for viewing the images formed by erecting lens 11. It will be noted that lenses' 15 are positioned at substantially the correct interocular distance apart to give the device substantially the same binocular angle as if the user viewed the field directly.

In the use of this device, an electric field is provided within the space 13, asindicated by the lead wires in Fig. 7, effective to draw electrons from the layers 66 and 14 directly across the space 13 to the screen 61 in relatively narrow parabolas, depending upon their respective angles of emission from the emissive layers, voltagedividing resistors 18 'and 19 being provided as shown for holding layer 14 at a suitable potentialY between those of layer 66 and screen 61. As a result, the electrons emitted from any point on the emissive layers will fall Within a relatively small region on the screen and will thus produce an image of amplified brightness and sumcient sharpness for adequate viewing. Control and saturation of amplification may be obtained with this system by constructing one or more of the emissive layers 66 and 14 as describedin connection with Figs. 2 to 5, or by equipping the screen 61 as described in connection with Fig. 6. I t will also be understood that the operation will be substantially the same lif a pair of amplifiers 65 is used, with each providing an image for one eye of the user. .1.

. It is also possible in accordance with. the invention to utilize an optical system which is folded back on itself in an arrangement roughly comparable to that of a prism binocular. Fig. 8 illustrates such an arrangement wherein. the surface 80 represents a source of lelectron emission, which may be a source of` primary emission arranged for actuation by incident light from the field as indicated by the arrows 8| or a source of secondary emission arranged for actuation by electrons from a suitable primary source, in which case the surface will correspond to the emissive layer 22 in Fig. 2. The electrons emitted by the surface 80 are focused by the electron lens 82 on a further electron emissive surface 83, which is shown as arranged to emit electrons from its surface facing the lens 82. These secondary electrons are in turn focused by an electron lens 84 on a viewing screen 85, and the resulting image may be viewed as indicated by means of a suitable erecting eyepiece 86, a suitable Venvelope or Vhousing for this system being indicated at 88. It will be readily apparent that this arrangement of multiple electron emissive layers may be carried to further lengths to build up the number of electron sources and thus the maximum amplification. Any one or morevof these electron sources may be constructed as described in connection with Figs. 2 to 5 to provide the desired saturable control over maximum brightness.

It will accordingly be seen that the invention provides an optical system which may be utilized in a viewing device under substantially all conditions of illumination to give adequate visibility without the danger of glare or blinding as a result of overly brightl light sources in the eld. With the device of the invention, the light from the field of view will be amplified to give an image wherein all parts of the field will be clearly visible and in approximately normal relativek optical contrast with the exception of bright light sources, which will be reduced in relative intensity to an extent suicient to prevent interference with vision of the remainder of the image. It may be used not only at night, when normal illumination is inadequate for unaided vision, but also when there is ample light but a glare source, for example when driving toward the setting sun or particularly when the sunlight is reflected toward the driver from a flat surface. If the photoelectric material in the receiving means of the device is chosen so as to be sensitive to infrared light, the device Yof the invention will be found highly useful for driving in fog, particularly in an automobile having head lights of sucient wattage to provide an adequate source of infrared radiation and with filters either over the headlights or over the device of the invention to cut out the visible light scattered by the mist, and accordingly the terms light and light rays are used herein to in. clude both visible light and also similar radiant energy such as ultraviolet and infrared radiation of wavelengths close to the visible range. While the forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is` not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

' What is claimed is: l,

1. An optical device for producing nonlinear optical amplification comprising, in combination, means forV receiving light rays from a field of View, means actuated by said receiving means to produce a visual image of said field in response to light rays incident from said field on said receiving means, means cooperating with said receiving and image-producing means for lamplifying the brightness 'of said image with relation to that of said field and for causing predetermined alteration of the ratio Yof brightness of the' relatively `more and less bright .areas in said im'- age with respect to the ratio of brightness of the corresponding areas in said field, and said device including a layer of material .adapted in response to radiation incident thereon to emit electrons at a ratio of emitted to incident radiation for veach unit area thereof varying with the rate of emission of electrons therefrom to effect said altera-tion of the brightness vof :said image with respect to the -brightness of said field.

2. An optical device for producing nonlinear optical amplification comprising, 'in combination, means for receiving .light rays from a Vfield vof View., means actuated .by said .receiving means to produce a visual image of .said field in response to light rays .incident from said field on said receiving means, 'and means cooperating with Vsaid receiving and -ir'nageproducing means for amplifying the brightness of said image with relation to that of said field and for causing predetermined decrease of the ratio vof brightness of the relatively more and less bright .areas in said iinage with respect tothe 'ratio of brightness of the corresponding areas in said field to provide reduced optical contrast but amplified 'general brightness Ain said image as compared with said field for clear visibility of each area in said image substantially free from glare, said device including a layer of material adapted in response to radiation incident thereon to emitelectrons ata ratio of emitted to incident .radiation for each unit area thereof which decreases progressively with increase in 4the rate 'of electron emission therefrom through at least a part of the emissivity range thereof to effect said reduced optical contrast in said visual image.

3. An optical device for producing nonlinear optical -amplification comprising, in combination, means for receiving light rays from a field of view, means actuated by said receiving means to produce a visual image of said field in response to light rays incident from lsaid field ori said receiving means, means lfor amplifying the brightness of said image with respect to the brightness of said field, means cooperating with said amplifying means to cause predetermined increase in the ratio of brightness of the .relatively more and less bright areas in said image with respect to the 'ratio of brightness of the corresponding areas in said field, said ainplifying means including a layer of material `adapted -to emit electrons in response to radiation incident thereon, and said cooperating means including a layer of material positioned in the path of said electrons from said amplifying means and adapted in response to radiation incident thereon to emit electrons at a ratio 'of emitted to incident radiation for each unit area thereof which increases progressively With increase in the rate oi electron emission therefrom through Aat least a part of the emissivity range thereof to effect said increased optical contrast in said visual image.

4. A viewing device of the character described for producing nonlinear optical amplification comprising, in combination, a mounting, a, pair of means for receiving light rays from a field of view positioned in laterally spaced relation in said mounting, means actuated by each of said receiving means to produce a visual image of said field in response to light rays incident from 'said field on said receiving means, said image-reproducing means being positioned in laterally spaced relation in said mounting for binocular vision by a user ef said device, `means for amplifying the brightness of said image relative to the bright-v ness of said field, and means for causing predetermined alteration of the optical contrast of said image with relation to said field.

5. A viewing `device of the `character described for producing nonlinear optical .amplifica-tion comprising, in combination, an ophthalmic mounting, a pair of photoelectric means `for ireceivin'g light .rays 'from a field of view positioned in laterally 'spaced relation in said mounting, means actuated by each of said receiving means to produce 'a visual image of said field in .response to light incident from said field on .said receiving means, means for amplifying the brightness of s'aid images relative to the .brightness of .said eld, means 'for limiting amplification by said amplifying .means and providing a .maximum brightness per .unit :area "of said images, 'and means positioning said image-reproducing .means in Isaid mounting and in laterally spaced relation for binocular vision by a user of 'said device.

6. .An optical device of vthe character described for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a field of view, means actuated .by said .receiving means to .reproduce an optical 1mage of said field in response to light rays incident from said eld on said receiving means, means for amplifying the brightness 'of said image. rela tive to the brightness of said field, lsaid device including a layer of electron emiss'ive material, :and means limiting the emission per unit karea from said emissive layer to prevent 'the brightness of any portion of said image from exceeding a predetermined maximum.

7. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, means for .receiving -light rays from a field of View, means actuated by said receiving means to reproduce an optical image of said field in response to light rays Vincident from said field on said vreceiving means, means for amplifying the brightness of said image relative to the brightness of said field, said -device including a layer of electron emissive m-aterial, and means for providing selective electrical opposition to the flow of electrons from said emissive layer in `accordance with the distribution of brightness in said field and to cause reduced optical contrast in said image.

8. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a field of view, means actuated by said receiving means to reproduce an image.

of said field in response to light rays incident from said field on said receiving means, means for amplifying the brightness of said image relative to the brightness of said eld, one of said receiving and amplifying means including a layer of elec-tron emissive material comprising a multiplicity of discontinuous areas to limit the fiow of electrons from each said area and to cause reduced optical contrast in said Aimage relative to said field.

9. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a field of view, means actuated by said receiving means to .reproduce an image of said field in response to light rays incident from said field on said receiving means, means for amplifying the brightness of said image relative to the brightness of said field, one of said receiving and amplifying means including a layer of felec-` T tron emissive material comprising a multiplicity ofv discontinuous areas to limit the flow of electrons from each said area yand to cause reduced optical contrast in said image relative to said iield, and means positioned between said emissive layer and said image-reproducing means and eiective to develop a negative charge in response to electrons incident thereon to provide selective electrical opposition to the ilow of said electrons to said image-reproducing means in-'accordance with the distribution of brightness in said ield.

10. An optical device of the character described for producing nonlinear optical amplication comprising, in combination, means including a layer of photoelectric material for receiving light rays from a field of view, means actuated .by said receiving means to reproduce an image of said field in'response to light rays incident from said eld on said receiving means, means for amplifying the brightness of said image relative to the brightness of said field and including a layer of electron emissive material positioned between said photoelectric layer and said image-reproducing means and adapted to provide secondary electrons in response to electrons incident thereon from said photoelectric layer, said device being constructed and arranged to provide an image of reduced optical contrast but increased -brightness relative to said eld.

11. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a field of view, means actuated by said receiving means to reproduce an image of said field in response ,to light rays incident from said eld on said receiving means, means for amplifying the brightness of said image relative to the brightness of said eld and including a pair of layers of electron' emissive material cooperating to emit secondary electrons in the direction of said image-reproducing means in response to light rays incident from said field on said receiving means, and means for maintaining the potential difference between said emissive layers such that the emission of electrons by the one of said pair of emissive layers nearer said imagereproducing means is in excess of the number of electrons incident on said layer from the other layer of said pair to produce on said first named layer selectively arranged net positive charges corresponding to brightly illuminated areas in said eld, with resulting increased effective emis- `sion from said other layer and increased relative brightness in the corresponding areas of the final image.

12. An optical device of the character described for rproducing nonlinear optical amplication comprising, in combination, means including a layer of photoelectric material for receiving light rays from a field of View, means actuated by said receiving means to reproduce an image of said field in response to light rays incident from said iield on said receiving means, means for amplifying the brightness of said image relative to the brightness of said iield and including a layer of secondary electron emissive material positioned lbetween said photoelectric layer and said image reproducing means and adapted to provide secondary electrons in response to electrons incident thereon from said photoelectric layer, one of said electron emissive layers comprising a multiplicity of discontinuous areas, each said area being adapted to become saturated when the light from the corresponding portion of said eld exceeds a predetermined maximum brightness and thereby F16 to prevent the brightness of the corresponding portion of said image from exceeding a predetermined maximum.

13. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a eld of view, means actuated by said receiving means to reproduce an image of said eld in response to light rays incident from said eld on said receiving means, means for amplifying the brightness of said image relative to the brightness of said eld, said receiving and amplifying means including a thin layer of electron emissive material, the thickness of said layer being such that when the flow of electrons from a -portion thereof exceeds a predetermined maximum, said portion will acquire a relatively positive charge and further electron emission there-- from will belimited accordingly to reduce the optical contrast of said image relative to said field.

14. An optical device of the character described for producing nonlinear `optical amplification comprising, in combination, means for: receiving light rays from a field of view, means actuated by said receiving means to reproduce an image of said field in response to light rays incident from said eid on said receiving means, means for ainplifying the brightness of said image relative to the brightness of said eld, said receiving and amplifying means including a composite of a supporting layer of substantially transparent materialrhaving comparatively high electrical resistance, a layer of electron emissive material on one side of said resistance layer, and a layer of substantially transparent electric conducting material on the other side of said resistance layer, said resistance layer being of such thickness and resistance as to cause said electron emissive layer to acquire selectively distributed positive charges in accordance with the corresponding distribu-v tion of brightness in said field.

15. An optical device of the character described for producing nonlinear optical amplication comprising, in combination, means including a l-ayer of photoelectric material for receiving light rays from a eld of view, means actuated by said receiving means to reproduce an image of said eld in response to light rays incident from saidv field on said receiving means, means for amplifying the brightness of said image relative to the brightness of said field and including a layer of secondary electron emissive material positioned between said photoelectric layer and said image-- reproducing means and adapted to provide secondary electrons in response to electrons incident thereon from said photoelectric layer, said layer being positioned closely adjacent said image-reproducing means, an-d a layer adjacent said sec-l ondary emissive layer on the side away from said image-reproducing means comprising material having a potential in the quiescent state above that of said secondary emissive layer and said` image-reproducing means and adapted to acquire a Vselective positive charge in response to electrons incident thereon from said secondary emissive layer to reduce the flow of said electrons to said image in accordance with Vthe brightness of said field.

16. An optic-al -device for lproducing nonlinear optical amplification comprising, in combination, means for receiving light rays from a fieldof view, -amplifying means actuated by said receiving means in response to light rays incident on said receiving means from said field to form an electron image of said field of amplified brightness with respect to said field, means for converting said electron image into a visual image of amplifled brightness `with respect to said field, and said amplifying means including a layer of material adapted in response to electrons incident thereon to emit secondary electrons at a ratio of emitted to incident electrons for each unit larea thereof varying with the rate of emission of electrons therefrom to effect predetermined alteration of the optical contrast of said visual image with respect to said field.

17. An optical device for producing nonlinear optical amplification comprising, in combination, means for receiving light rays from a field of view, means cooperating with ,said receiving means to form an electron image of -said field of amplified brightness with respect to said field, means actuated by said electron image means to form -a visual image of said field, each of said image-forming means including a layer of material adapted to emit electrons in -response to radiation incident thereon, and means establishing a ratio of emitted to incident radiation for each unit farea of at lleast one said layer which decreases progressively upon increase in the rate of emission of electrons therefrom to effect correspondingly progressive reduction in the optical contrast in said visual image with respect to said field.

18. An optical device for producing nonlinear optical amplification comprising, in combinationr means for receiving light rays from a field of view, means for producing ia visual image of said vfield in response to light rays incident from said field l on said receiving means, and means-for amplifying the brightness of said image with respect to the brightness of said field including a layer of material adapted in response to vradiation incident thereon to emit secondaryelectrons 'at a .i

ratio of emitted to incident electrons'for each y unit area thereof which varies depending upon optical amplification comprising, in combination, y

means for receiving light rays from a field of view, means actuated by said receiving means to produce a visual image of said fieldfof amplified brightness with respect to said fieldin response to light rays incident from said field on said receiving means, said device including a layer of material adapted to emit electrons in response to radiation incident thereon, and means establishing a progressive decrease in the ratio of emitted to incident radiation for each unit area of said layer to provide correspondingly decreasing amplification of brightness in said image resulting in reduced optical contrast in said image wth respect to said field of view.

20. An optical device of the character described for producing nonlinear optical amplification comprising, in combination, photoelectric means for receiving light rays from a field of view, means actuated by said receiving means to reproduce an image of said field in response to light rays incident from said field on said receiving means, means for amplifying the brightness of said image with respect to the brightness of said field including a layer of material adapted to emit secondary electrons in response to electrons incident thereon at a ratio of emitted to incident electrons for each unit larea thereof decreasing with increase in the rate of emission of electrons therefrom to cause progressive reduction in the-degree of amplification of the brightness in the image formed thereby and corresponding reducition in the optical contrast in said visual image with respect to said field.

21. A viewing device of the character described for producing nonlinear optical lamplification comprising a mounting, means in said mounting for receiving light rays, from Aa field of view, means in said mounting actuatedA by said receiving means to produce a visual image of said field in response to llight rays incident from said field on said receiving means, means positioned in said mountingbetween said receiving means and said image producing means for amplifying the brightness of said image with respect to the brightness of said field, means in said mounting cooperating with said amplifying means to cause predetermined alteration of the optical contrast of said image with relation to said vfield, and lens means in said mounting providing for direct viewing of sai-d visual image.

CLINTON J. T. YOUNG.

References Cited in the file of this patent UNITED STATES PATENTS 

